We have found that a large variety of agents can stimulate DNA synthesis in cultures of chick embryo fibroblasts. These agents also stimulate a wide array of metabolic pathways in the cells including their differentiated functions. We have been conducting a search for an underlying reaction which is capable of regulating many independent cellular activities, and this past year we have produced evidence that this reaction is the activation of magnesium (Mg) in the cell. This year we shall be measuring the concentrations of Mg, Ca, Na and K in cells under various conditions. We shall also be using enzyme equilibrium measurements to estimate the free Mg in cells. We shall also be determining whether Mg deprivation affects precisely the same biochemical steps as does removal of serum, low pH or high population density. BIBLIOGRAPHIC REFERENCES: Rubin, H. and Steiner, R. Reversible alterations in the mitotic cycle of chick embryo cells in various states of growth regulation. J. Cell. Physiol. 85: 261-270, 1975. Fodge, D.W. and Rubin, H. Glucose utilization, pH reduction and density dependent inhibition in cultures of chick embryo fibroblasts. J. Cell. Physiol. 85: 635-642, 1975.